Mars Express captures impact scars on the Red Planet
These elongated craters are probably the result of two projectiles slamming into the same area.
March 7, 2011
The European Space Agency’s (ESA) Mars Express has returned new images of an elongated impact crater in the southern hemisphere of Mars. Located just south of the Huygens basin, it could have been carved out by a train of projectiles striking the planet at a shallow angle.
Elongated crater on Mars.
Photo by ESA
The large Huygens basin is about 280 miles (450 kilometers) in diameter and lies in the heavily cratered southern highlands. In this area, there are many impact scars but none perhaps are more intriguing than the elongated craters.
One of these craters is seen in this new image, which covers an area of 83 by 33 miles (133 by 53 km) at 21° S/55° E. The scene was captured August 4, 2010, and the smallest objects distinguishable by the camera are about 16 yards (15 meters) across.
This unnamed elongated crater sits just to the south of the much larger Huygens basin. It is about 48 miles (78 km) in length, opens from just under 6 miles (10 km) wide at one end to 16 miles (25 km) at the other, and reaches a depth of 1 mile (2 km).
Impact craters are generally round because the projectiles that create them push into the ground before the shock wave of the impact can explode outward. So why is this one elongated?
The clue comes from the surrounding blanket of material that was thrown out in the initial impact. This “ejecta blanket” is shaped like a butterfly's wings, with two distinct lobes. This hints that two projectiles, possibly halves of a once-intact body, slammed into the surface here.
In the crater itself, there are three deeper areas that could be evidence for more than two projectiles. In addition, a second elongated crater lies to the north-northwest. It is in line with the one seen here, reinforcing the notion that these structures were the result of a train of projectiles.
In the early 1980s, scientists proposed that incoming chains of orbital debris following trajectories that decayed with time formed elongated impact craters. As the debris spiraled downward, it eventually struck the planet at shallow angles, gouging out the elongated craters.
This particular ejecta blanket contains many smaller craters, indicating that the original formed a relatively long time ago and then itself become a target.
In addition, there are several small channels on the blanket, suggesting that the strike took place into a surface rich in volatiles, perhaps even water, that were melted by the heat of impact and flowed away.
Below the eastern crater rim are two well-formed and relatively deep craters. They have punched through the ejecta blanket and so must have appeared after the formation of the large crater. Despite their sizes of 2 miles (4 km) and 3 miles (5 km), these smaller craters show no indication of water.
To the north there is another crater that must be older because the butterfly-ejecta blanket has partially flowed into it.
Several landslides have modified the steep crater rim. This can be clearly seen on the two smaller craters on the rim, which are only partially preserved, parts of them having fallen away.
The formation of these elongated features is not over. The martian moon Phobos will plough into the planet in a few tens of millions of years, breaking up in the process, and likely creating new chains across the surface.